1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire, and more particularly to an improvement of service durability in the bead portion of the heavy duty pneumatic radial tire for use in airplane wheel or the like forcedly subjected to large longitudinal deformation during the running under heavy loading.
In the case of tires for airplanes, the large longitudinal deformation produced in the tire during the running under heavy loading is particularly conspicuous in the taxiing of the airplane, which strongly affects the service durability of the bead portion.
2. Related Art Statement
In this type of the conventional pneumatic radial tire, a toroidal carcass ply is wound around a bead core from the inside of the tire toward the outside thereof to form a turnup portion in a bead portion. The reinforcement of the bead portion is generally performed by embedding a stiffener, a main chafer and two auxiliary chafers in a bead portion rubber. In this case, the stiffener is disposed between the carcass ply and its turnup portion and consists of a base portion placed on the bead core and made of hard rubber stock and a thickness-reducing portion integrally united with the base portion and made of soft rubber stock. Further, the main chafer is arranged outside the carcass so as to extend from a level lower than the upper end of the turnup portion along the turnup area of the carcass ply toward the inside of the tire, while the auxiliary chafer has a modulus of elasticity lower than that of the main chafer and is extended downward from a level higher than the upper end of the turnup portion so as to cover the upper end of the turnup portion and one end of the main chafer. Moreover, the two auxiliary chafers may be arranged outside the main chafer or so as to sandwich the upper end of the turnup portion and one end of the main chafer therebetween, wherein the cords of the auxiliary chafers are crossed with each other at the same inclination angle with respect to the meridional plane of the tire.
Since the longitudinal deflection of the tire is conspicuous in the taxiing of the airplane, a large deformation is forced to the bead portion. In the aforementioned conventional bead portion reinforcement, therefore, fatigue has been experienced that is considerably accumulated in the bead portion due to the repetitive bending stress at ground contact area during the rotation of the tire. Separation failure frequently occurs in the boundary between the carcass turnup end and the rubber surrounding therearound.
Therefore, it has been attempted to reduce strain produced in the bead portion by using three auxiliary chafers and making the cord angles of these auxiliary chafers different to form a so-called triangular truss structure, but such an attempt is still insufficient.
For example, there is a pneumatic radial tire as disclosed in U.S. patent application Ser. No. 826,159, now U.S. Pat. No. 4,700,764, wherein a belt as a reinforcement for tread portion is superimposed about a crown portion of a radial carcass of two cord plies each containing aromatic polyamide fiber cords (3000 d/3) therein and composed of seven belt cord layers each containing the same aromatic polyamide fiber cord layers (3000 d/3). The cords of second and fourth layers viewed from the inside are crossed with each other at an inclination angle of 23.degree. with respect to the equator of the tire and the cords of the remaining layer are arranged substantially in parallel with the equator of the tire. The main chafer is composed of a cord layer containing aromatic polyamide fiber cords (3000 d/3) arranged at an inclination angle of 60.degree. with respect to the cords of the carcass upward to the right. The auxiliary chafer is composed of three cord layers each containing nylon cords (1260 d/2) in such a manner that the cords of a first outer cord layer viewed from the outside of the tire are arranged at an inclination angle of 75.degree. with respect to the meridional plane of the tire upward to the right. The cords of a second outer cord layer are arranged at an inclination angle of 65.degree. downward to the right, and the cords of a third inner cord layer are arranged at an inclination angle of 45.degree. upward to the right. In case of using such a tire for an airplane, the satisfactory service durability is obtained under normal service conditions, but cracks are produced at the end portion of the third inner cord layer under severer conditions such as a load heavier by at least two times than the normal load, a state after the running over long distance and the like.
Further, in the tire having the same structure as mentioned above except that the inclination cord angle of the third inner cord layer is 65.degree., the satisfactory service durability is obtained under normal service conditions, but cracks are produced at the turnup end of the carcass ply under the severer conditions.